All tarantulas are carnivorous, and they feed on living prey. Most tarantulas seem to prefer small insects and other invertebrates that don’t put up much of a fight when caught, which means the tarantulas get dinner by expending very little energy. Even the largest tarantulas would prefer to take half a dozen nearly defenseless crickets rather than have to fight with a single armored beetle that weighs the same as the combined weight of the crickets.
There are indeed records of wild tarantulas actually taking nestling birds and small rodents from their nests, as well as the occasional lizard, frog, and even small snake, but these are exceptions to the usual rule. Some tarantulas will take animals that have recently died, but in most cases they can only detect food when it moves, so a dead cricket will be ignored unless the mouth of the tarantula is actually placed over it.
(This sometimes is done with very ill tarantulas that can only eat squashed, freshly killed crickets.) The fangs inject a neurotoxin that can kill prey up to the size of a mouse but generally is harmless to humans. A very large tarantula has tremendous force in its chelicerae, however, and can drive a fang through a human fingernail.
Digestion takes place outside the body, with the spider regurgitating strong digestive juices on the prey, which usually is wrapped in silk to help control it and also to store it for later meals. Remember, the tarantula does not actually swallow its food—what you see left in the terrarium are empty husks of crickets with all their soft tissues digested away. If you feed a very juicy morsel, such as a pinky mouse (a newborn mouse before its eyes open), to a tarantula, the spider will treat it just like a cricket, including mangling it by the maxillae of the pedipalps and producing a truly bloody mess. The remains will be shriveled skin bits with some cartilage and other indigestible parts.
Tarantulas feed at night, as a rule, generally waiting for prey to wander near the opening of the burrow or scrape and trip a fine silk line that warns the spider that prey is near. When motion is detected close enough (less than a body length, usually) to the tarantula, the spider rapidly moves from the burrow, grabs the prey with its pedipalps and front legs, and injects it with venom that kills it or at least slows its heartbeat enough to prevent struggling.
The prey may be eaten immediately or, if food is abundant, wrapped in silk and stored for later. A tarantula will feed until it is sated and will ignore prey if it is already full. Because of the low energy requirements of a tarantula, it needs little food, and an adult female tarantula may reproduce and survive on only a few insect meals each month during the active season.